That reminds me ... To a mathematician, the statement Proof: obvious means, If you've been following what's going on so far then I really don't need to spell this out for you. One day, in a maths lecture, a professor writes a minor theorem on the board and adds, "Proof: obvious." Then he stops, looks, thinks, says, "Is it really obvious?", thinks some more, spends 5 minutes scribbling on some scrap paper and muttering to himself, and finally says, "Yes, it is obvious." and resumes the lecture as if nothing had happened.
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PitarouJan 12 '12 at 17:05

+1, this is a very tasteful but direct "comeback", without the sensationalism of being clever or smart-ass.
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tenfourApr 26 '11 at 8:57

1

+1 for "Obviously?" Extensions along this line could be things like "Obviously? How so?" "I find things less clear. What about [this]?"
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MrHenApr 26 '11 at 16:10

3

+1 for "Obviously?" but the second suggestion is obnoxious. Sounds like a good way to end the civilized part of the discussion. To me it would reflect very badly on whoever said it.
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Jason OrendorffApr 26 '11 at 17:24

Someone overeager to sound intelligent is likely to enjoy going into depth. Simply asking "What do you mean?" or "Can you explain it further?" is a good way to keep them talking. If the details in their obvious point start breaking down it should open the discussion up for a direct counterpoint or counterexample:

Obviously blah blah.

What do you mean?

Blah blah blah.

What about [example]?

Blah blah blah.

And [point]?

[Backpedalling goes here.]

The real wit here is that a group of intelligent listeners will likely notice the inconsistency before the speaker does. If they care enough, they will continue the line of questioning for you and the original statement will be dismissed as a falsehood.

The right response depends crucially on what you want to achieve. The most common mistake people make in this situation is to miss the big picture and want the wrong thing.

You asked for a comeback, which suggests that you want to look good, gain the admiration of other listeners, and puncture the speaker’s ego. I don’t know the context, but I’m guessing what you should want in this situation is either to quit wasting time in an argument that nobody is going to learn anything from, or else to cool off the discussion and get it pointed in a productive direction.

To do that, first check to make sure your body language, facial expressions, and tone are friendly and not confrontational. Then, ignore the word obviously and politely address the substance of the argument instead.

(This question is tagged repartee, which suggests a totally different goal. But repartee is a witty exchange; it takes two to tango, and your stereotypical obviously speaker is not going to be a good dance partner. Judging by these answers, a global community of StackExchange users is not a very good dance partner, either. Zing! ;-)